A conventional open-control daisy wheel printer has a construction comprising, as shown in FIG. 1, a platen 1, a carriage assembly 2 for determining a printing position, a belt 3 for driving the carriage assembly 2, a daisy wheel 5 having types 5a, 5b, 5c . . . respectively formed at the ends of spokes, a stepping motor 6 for rotating the daisy wheel, and a hammer 7 for applying printing pressure to the type. In such a printer, the printing position is determined by the carriage assembly 2, the daisy wheel 5 is rotated by the stepping motor 6 to select the printing type, and then the printing type at one end of a spoke is struck on the back side thereof by the hammer 7 and caused to hit the printing paper 8 on the plten 1 whereby its impression is printed thereon.
In a printer of this kind, in order to confirm whether or not the printing type is accurately selected (hereinafter called "detection of deviation"), it is conventional for a reference position of the daisy wheel 5 to be determined in correspondence with a given type (for example, "W" which is hereinafter called "a reference position character") so that detection of deviation is carried out each time "W" is printed.
In more detail, conventional detection of deviation is carried out by employing the construction shown in FIG. 2. In these drawings, like parts are denoted by the same reference numerals. In the construction of FIG. 2, a sensor device 13 comprising a light emitting element 11 and a light receiving element 12 is fixed securely on the carrier assembly 2. Further, secured on a rotary shaft 15 of the stepping motor 6 is a sensor bar 16 which is so positioned as to obstruct the light path of the sensor device 13 when said reference position character (for example, "W") is selected to occupy the position to be struck by a hammer stem 17 of the hammer 7. FIG. 3 shows the sensor bar 16 in a perspective view. The sensor bar 16 rotates integrally with the rotary shaft 15.
In other words, the stepping motor 6 rotates the daisy wheel 5 sequentially in response to printing character information and sequentially selects types in correspondence with the printing type information. In the absence of deviation, when the daisy wheel 5 is rotated to select the type "W" in correspondence with the printing type information .cent.W", the sensor bar 16 obstructs the light path and, therefore, a sensor signal is outputted by the sensor device 13. In the presence of deviation, however, even when the stepping motor 6 rotates the daisy wheel 5 in correspondence with the printing character information "W", the type "W" is not selected and the sensor bar 16 does not obstruct the light path of the sensor device 13. Therefore, the sensor signal is not outputted, whereby the deviation is detected.
In the conventional deviation detecting device of this kind, however, detection of deviation is only performed when said reference position character is printed. Accordingly, the conventional deviation detecting device of this kind has the disadvantage that it cannot carry out detection of deviation in the case of printing continuously printing character information which does not include said reference position character.
Further, in the conventional deviation detecting device, paper feed immediately after turning on a power source switch 20 (shown in FIG. 4) causes misalignment of printing lines due to deviation in position between a rotor and an exciting coil of a paper feed motor (not shown) and/or backlash of gears for transmitting a turning force to the platen. That is, the conventional deviation detecting device has the disadvantage that the interval between adjacent printing lines can lose its uniformity, as shown by 21 in FIG. 4, to thereby cause misalignment in printing lines.
In order to obviate this disadvantage in the conventional printer, compensation for this misalignment (hereinafter referred to as "paper feed compensation") is carried out immediately after switching on the power or the acceptance and execution of a reset command from the outside. That is, printing is performed after the paper feed has been adjusted by forwardly rotating the paper feed motor by several steps and then reversely rotating it by the same number of steps.
In performing a printing operation with such a printer, it is conventional to stop the printer printing (hereinafter called the "pause38 mode) even after turning on the power, or to manually operate a platen knob 22 to thereby determine the printing line position or the printing page.
However, in the case where the paper feed device is driven manually with the printer in pause mode, the conventional device shows a disadvantage of misalignment in printing lines because the paper feed device is not adjusted (no compensation for the misalignment is made).
Accordingly, an object of the present invention is to provide a printer which is capable of performing detection of deviation of the daisy wheel even when continuously printing a series of printing character information which does not include a reference position character.
Another object of the present invention is to provide a printer which is capable of automatically adjusting the paper feed device by paper feed compensation even immediately after termination of a pause mode of the printer.